The optical wavefront measurement has become one of the most important aplications in optical system. Commercially available optical wavefront measurement apparatuses can be divided into two groups. One is a different path interferometer represented by Ladite Machine of American Wyko company; the other is a common path interferometer represented by WaveAlyzer of American Blue Sky Research. The common path interferometry technique has gradually become a mainstream due to its advantage of protecting the system from environmental interference.
Please refer to FIG. 1 which is a schematic diagram showing a phase-shearing interferometry technique. When a light beam I is made incident on a phase shifter 11, two reflected beams R1, R2 generated from the front and back surfaces of the phase shifter 11 and having a path difference attributed to a thickness t of the phase shifter 11 are interfered with each other to produce an interference pattern. Please refer to FIG. 2 which shows a hardware structure of the known WaveAlyzer arranged with two sets of phase shifters 21, 24 and reflective mirrors 22, 25 for directing an incident light beam I into the charge coupled devices (CCD) 23, 26 adapted to be used as cameras so as to receive an interference pattern. One of the phase shifters 21, 24 is disposed ahead of the other by a phase of 90.degree. with respect to the incident directon of the light beam I.
Please refer to FIG. 3 which is a schematic diagram showing a flowchart of an optical wavefront measurement based on a phase-shearing interferometry technique. The phase shifter 21, the reflective mirror 22 and the CCD 23 are used to receive an interference pattern from the first axis (called as X-axis). The phase shifter 21 is finely rotated five times by the ThermX.TM. electronic driving device 27 for changing the incident angle .theta. of the incident light beam I with respect to the phase shifter 21 and a five-step phase shifting technique is adopted to obtain a first set of interference patterns. The light intensity function is expressed as: EQU I(x,y)=I.sub.0 (x,y)[1+.gamma..sub.0 cos .phi.(x,y)] (1)
where I.sub.0 (x,y) is the light intensity of the incident light beam I, .gamma..sub.0 is the visibility of an interference pattern, and .phi.(x,y) is the difference of the phase angle owing to shearing effect. There are three unknow variables (I.sub.0, .gamma..sub.0, .phi.) in Equation (1). In other words, at least three independent measuring results are needed to determine the difference of the phase angle .phi.(x,y). The Five-step technique is commonly used in the operational theory of light intensity phase transform. The phase shifter 21 is finely rotated five times by the ThermX.TM. electronic driving device 27 for changing the incident angle .theta. of the incident light beam I with respect to the phase shifter 21, resulting in changes in the intensities of interference lights received by the CCD 23. In a phase reconstruction process, a shearing interference technique is utilized for introducing a known phase shift into an intereference pattern so as to obtain five light intensity equations: EQU I.sub.1 (x,y)=I.sub.0 (x,y)[1+.gamma..sub.0 cos(.phi.-2.alpha.)](2.1) EQU I.sub.2 (x,y)=I.sub.0 (x,y)[1+.gamma..sub.0 cos(.phi.-.alpha.)](2.2) EQU I.sub.3 (x,y)=I.sub.0 (x,y)[1+.gamma..sub.0 cos(.phi.)] (2.3) EQU I.sub.4 (x,y)=I.sub.0 (x,y)[1+.gamma..sub.0 cos(.phi.+.alpha.)](2.4) EQU I.sub.5 (x,y)=I.sub.0 (x,y)[1+.gamma..sub.0 cos(.phi.+2.alpha.)](2.5)
where .alpha. represents the relative phase shift obtained by finely rotating the phase shifter 21 resulting in a changed difference of the phase angle. From Eqs. (2.1).about.(2.5), after a few derivations, we obtain: ##EQU1## when .alpha.=.pi./2, Eq. (3) can be simplfied as: EQU tan.phi.=2(I.sub.2 -I.sub.4)/(2I.sub.3 -I.sub.5 -I.sub.1) (4)
By the above-mentioned method, the interference of random noise can be eliminated and the difference of the shearing phase angle .phi.(x,y) can be obtained immediately and accurately.
The phase shifter 24, the reflective mirror 25, and the CCD 26 are used to receive an interference pattern from the second axis (called as Y-axis). The phase shifter 24 is also finely rotated five times by the ThermX.TM. electronic driving device 27 for changing the incident angle .theta. of the incident light beam I with respect to the phase shifter 24 and a five-step phase shifting technique is adopted to obtain a second set of interference patterns and the difference of the shearing phase angle .phi.(x,y).
A phase unwrapping process is then performed for the two sets of interference patterns from the first axis and the second axis, respectively, through a function fitting method with programs installed inside the computer 28 so as to complete an optical wavefront inspection.
The above-mentioned conventional technique has two shortcomings in practical application: one is that it is difficult to place a prior mechanism having two independent systems required for generating and receiving interference patterns into an experimental system for optical wavefront inspection due to the large size of the prior mechanism; another is the phase inconsistency caused by the existence of two phase shifters 21, 24 separated from each other by a certain distance. The phase inconsistency becomes particularly evident as the optical wavefront of a light beam becomes more sophisticated. Any environmental disturbance occurred between two phase shifters 21, 24 will introduce extra errors into the received data and spoil the consistency between data from two axes. Furthermore, since the data from two axes will be sent to the CCD 23, 26 at different positions, those data from two axes have to be adjusted into consistence.
Thus, it is tried by the applicant to deal with the situation encountered by the prior art. An optical wavefront analyzer having fewer components in comparison with the commercially available apparatus is developed. The size of the system according to the present invention is about one-tenth of other system performing similar functions. The small size of this system according to the present invention is advantageously connected to other optical experimental equipment and also greatly widens the application of optical wavefront measurement. In addition, the present invention overcomes the problems of two-axial focusing and phase inconsistency by employing phase-shearing and phase-shifting techniques to improve the method of optical wavefront mesurement.